The present invention relates generally to steam generators used in the production of steam for electric power generation and, more particularly, to method and apparatus for modifying an existing steam generator to accommodate the firing of different coals which have markedly different combustion characteristics, such as the resulting flue gas temperature when the coal is burned.
For a general description of boilers or steam generators used in the production of steam for industrial or utility applications, the reader is referred to Steam/its generation and use, 41.sup.st Edition, Kitto and Stultz, Eds., Copyright© 2005, The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein.
FIG. 1 illustrates a 1300 MW supercritical pressure, UP® steam generator designed and manufactured by The Babcock & Wilcox Company. Briefly, coal is ground to a desired fineness and conveyed to the furnace via burners which mix the pulverized coal with air in a combustion process. Hot flue gases created during combustion flow upwardly through the furnace. The furnace walls are of welded, membrane tube wall construction. The tubes forming the walls convey a working fluid therethrough which absorbs heat from the combustion process in order to produce steam. The flue gases flow from the furnace and across various banks of heating surface comprised of tubes. Secondary superheater and reheat superheater (pendant) are located at the upper portion of the steam generator. These banks of heating surface extract heat from the flue gases flowing there across, increasing the temperature of the working fluid conveyed within these tubes, while the temperature of the flue gases gradually decreases. The flue gases then travel into the convection pass, and thence downwardly across additional banks of heating surface which are also comprised of tubes which convey a working fluid; i.e., primary superheater, reheat superheater (horizontal) and economizer. The flue gases then exit from the steam generator and are conveyed to air heaters which extract additional heat from the flue gases which is used to preheat the incoming air which is used for combustion. Some of the air for combustion is used to dry and transport the pulverized coal from the pulverizers to the burners, and is referred to as primary air; the balance of the combustion air is generally referred to as secondary air and is conveyed to the burners via the forced draft fan(s). In the steam generator shown, an older version of steam temperature control is illustrated which utilized a combination of gas tempering ports and gas recirculating fans to distributed combustion flue gases at appropriate locations. As is known to those skilled in the art, other steam temperature control methods are known which do not utilize gas tempering and gas recirculation but otherwise the basic operational principles of such steam generators remains the same.
It will thus be appreciated that, in the production of electricity, various pieces of equipment are necessary. The boiler or steam generator is a combination of many pieces of equipment, which when combined use the heat released by the combustion of fossil fuels to heat the working fluid, typically water, and produce superheated steam. The steam has a large amount of energy, which is used to spin the blades of a turbine. The boiler fires a fossil fuel, such as coal, which produces the high temperature flue gas that passes across the several different types of heat exchangers which transfer heat from the flue gas into the water and steam system. The first heat exchanger where the water absorbs heat from the flue gas is the economizer.
FIG. 2 is a perspective illustration of a known economizer, generally designated 10, comprised of an inlet bank 12 and an outlet bank 14, and which would be typically located in the lower portion of a steam generator convection pass. Additional banks of economizer may also be provided, intermediate to the inlet and outlet banks, depending upon the requirements of a particular steam generator. The economizer 10 is comprised of hundreds of tubes 16 arranged in a serpentine pattern. An economizer inlet header receives water and distributes the water among the various tubes 16. The tubes 16 convey the water upwardly, counterflow to the direction of combustion flue gas flow, as shown, absorbing heat from the flue gases. At the water outlet (flue gas inlet) of the economizer outlet bank 14, intermediate headers 20 collect the heated water from the individual tubes 16, mixing out any imbalances in heat absorption. The intermediate headers 20, in turn, are connected to economizer stringer tubes 22 which convey the heated water upwardly through (inbetween) the tubes which comprise banks of other types of heating surface. As shown in FIG. 2, a horizontal primary superheater inlet bank 30 is located immediately above the economizer 10. The intermediate headers 20 serve several purposes. First, they serve as a mix point to eliminate imbalances in the temperature of the water which has been heated during its passage through the economizer 10. Second, the side to side spacing of the stringer tubes 22 can be selected to exactly accommodate the side to side spacing of the above located heating surfaces through which they pass; in the case shown, the side to side spacing of the primary superheater 30. In addition, the intermediate headers 20 can be located as required so that the stringer tubes 22, and the attached mechanical supports 24 hung off of the stringer tubes 22, and the non-cooled mechanical ladder bar supports 26, can be positioned as desired.
As described in the aforementioned Steam 41st reference, economizers are located within tube wall enclosures or within casing walls, depending on gas temperature. In general, casing enclosures are used at or below 850 F (454 C) and inexpensive carbon steel can be used. If a casing enclosure is used, it must not support the economizer. However, tube wall enclosures may be used as supports.
The number of support points is determined by analyzing the allowable deflection in the tubes and tube assemblies. Deflection is important for tube drainability. FIGS. 2, 3, 4 and 5 illustrate other economizer support arrangements.
As shown in FIG. 3, wall or end supports are usually chosen for relatively short spans and require bridge castings 40 or individual lugs 42 welded or attached to the tube wall enclosures 44. Vibration dampers 46 may be provided on individual tube banks to reduce flue gas flow induced vibration. As illustrated in FIG. 4, another possibility exists if enclosure wall (usually primary superheater circuitry) headers, such as lower convection pass enclosure wall headers 50, are present above the economizer 10. In this case, the support mechanism is again via non cooled mechanical supports, this time in the form of end support bars 52 which engage the ends of the tubes 16 forming the banks 12, 14 of economizer 10.
Quarter point stringer supports are used for spans exceeding the limits for end supports; this situation is illustrated in FIG. 5. The stringers 22 are mechanically connected at 24 to the economizer sections 14, etc., which are held up by ladder type supports 26. The supports exposed to hot inlet gases may be made of stainless steel, while lower grade material is normally used to support the lower bank which is exposed to reduced gas temperatures. In the B&W designs, stringer tubes 22 also usually support other horizontal convection surfaces above the economizer 10. Bottom support is sometimes used if the gas temperature leaving the lowest economizer bank 12 is low enough.
Economizers are thus generally supported in one of two manners depending on the enclosure surrounding the economizers. If the enclosure is a tube wall enclosure and the span of the economizer is not too long then the economizer is supported from the tube walls by bridge castings and support lugs. If the enclosure is casing and a primary or reheat superheat header is located above the economizer, non-cooled mechanical support ladder bars may be used for support.
The Babcock & Wilcox Company (B&W) has used the term stringer supported economizer in the past. However, in those designs the stringer tubes have not been routed through the economizer. Instead, as illustrated in FIGS. 2 and 5 discussed above, the actual support of the banks 12, 14 of economizer 10 used non-cooled mechanical ladder bar supports 26 which were connected via mechanical supports 24 to the economizer intermediate headers 20. The economizer intermediate headers 20 were then supported by the stringer tubes 22 in the upper elevations of the convection pass area where the flue gas temperatures are higher.
For many electric utilities, economics and emissions regulations have caused plant owners to switch fuels from the original design fuels. Steam generators are generally designed to accommodate a particular type of coal, which sets the furnace sizing and heat input parameters, the slagging and fouling indices, the coal pulverizers and associated burners, air heaters, etc. For a given furnace size and firing condition, the choice of fuel also determines the furnace exit gas temperature of the flue gas leaving the furnace and that temperature, as well as the gas weights, gas properties, and other heat transfer parameters are used to design the particular arrangement of superheater, reheater and economizer surface which will be provided. Combustion of a different coal in a steam generator which was not originally designed for that coal will usually result in different performance. In many instances, such a fuel switch often results in higher flue gas temperatures exiting from the furnace and such increased temperature profiles persist throughout the radiant and convective gas path, including the gas temperature entering the economizer. These higher temperatures can cause the traditional non-cooled mechanical support systems to become bulky and cost prohibitive. Accordingly, a cost effective, fuel flexible steam generator arrangement and a method of retrofitting existing steam generators which would provide such flexibility would be welcomed by industry.